Antibodies directed against the insulin receptor have been invaluable reagents in the investigation of receptor structure and function. Initially, these anti-receptor antibodies were identified as spontaneously appearing autoantibodies in patients with autoimmune disease associated with either extreme insulin resistance or hypoglycemia. The utility of these anti-receptor antibodies is limited to some extent because of several factors. Most importantly, it is not known which epitopes on the receptor are recognized by these antibodies. Consequently, we have recently attempted to develop antibodies which would recognize specific sites on the insulin receptor. Toward that end, we have immunized rabbits with peptides corresponding to partial amino acid sequences of the receptor. In addition, we have employed antibodies directed against pp60src to demonstrate structural homology between the insulin receptor and tyrosine kinases encoded by viral oncogenes.